If you needed to separate a glass container, for example, cut the bottom from a bottle, then you can use the simple way. To do this, you need to make several turns of a thin rope in the right place, and then soak the turns with combustible liquid. After giving a little burn, lower the bottle into cold water, if everything is done correctly, the bottle will crack in the right place.
From old bottles you can make plafonds, candlesticks and other crafts for which you need the upper or lower part of the glass product.